In recent years, SMART ENTRY ™ key systems (registered trademark) being capable of locking or unlocking doors of vehicles without using mechanical keys have been widely used. In SMART ENTRY ™ key systems, when a user carrying an electronic key approaches the vehicle, an on-vehicle device collates the ID code of the electronic key. When the validity of the electronic key is authorized as a result of the collation, the doors of the vehicle can be locked or unlocked without performing an opening or closing operation through insertion of a mechanical key into a cylinder lock of the vehicle.
At the time of authentication, the electronic key receives a request signal transmitted by the on-vehicle device. When receiving the request signal, the electronic key transmits a response signal including the ID code to the on-vehicle device. The on-vehicle device receives the response signal and performs authentication depending on whether there is a match between an ID code registered in advance and the ID code included in the response signal.
As the electronic key transmits or receives signals to or from the on-vehicle device, the remaining level of the rechargeable battery contained in the electronic key gradually gets low. For this reason, when the remaining level of the rechargeable battery falls below a predetermined value, the doors of the vehicle cannot be locked or unlocked and the engine cannot be started by using the electronic key.
Therefore, for example, a keyless device described in Patent Document 1 calculates a relative distance between an on-vehicle device and an electronic key through communications between the on-vehicle device and the electronic key as a mobile device. The keyless device stops communications for locking or unlocking the vehicle doors if the relative distance has exceeded a threshold.